Well-drilling fluids and especially completion, packer, and workover fluids are preferred in the form of clear salt aqueous solutions also commonly referred to as “clear brines”. Those fluids are called in accordance with the phases of the hydrocarbon extraction process in which they are employed: completion fluids are those used after the well has been drilled and prior to the initiation of production; packer fluids are utilized as fluids in the annulus of the production tubing; and workover fluids are used during remedial operations on the well. Of the multiple functions of the fluid used in the well, a crucial one is balancing formation pressures to prevent uncontrolled influx of underground fluids, which may result in a blowout. When formation pressure is high, as in the deeper wells, use of zinc bromide is particularly useful, due to its high density in concentrated solutions. While saturated brine density is 1.20 g/cm3 for NaCl solutions, 1.46 for CaCl2, 1.50 for NaBr and 1.71 for CaBr2, the density of ZnCl2 brine at saturation attains 2.14 and the density of ZnBr2 is as high as 2.65 g/cm3.
When in use, a technical problem associated with the use of heavy brine fluids is their high corrosiveness, especially toward carbon steels, which are widely used in the construction of hydrocarbon wells, and particularly at the elevated temperatures typically encountered in deep wells.
Known corrosion inhibitors, such as film-forming amines, which have been used with high-density brines described in U.S. Pat. No. 4,304,677 and U.S. Pat. No. 4,292,183 do not generally provide adequate protection from corrosion at those higher temperatures associated with the deep wells in which high-density brines are normally employed.
British Patent No. 2,027,686, U.S. Pat. Nos. 4,536,302, 4,728,446, 4,784,779 and 4,980,074 have disclosed use of sulfur compounds as the main inhibitor-formula component that has been typical of the recent known technology. Although the sulfur compounds are efficient corrosion inhibitors, there is an increased risk of stress corrosion cracking. There have been a number of catastrophic stress corrosion failures in the oil industry that have been attributed to sulfur containing corrosion inhibitors. As a result, the industry is seeking effective, non-sulfur corrosion inhibiting formulas.
Some non-sulfur corrosion-inhibition formulas are already known, but they fall short in respect to the effectiveness considered necessary in the industry. Thus, U.S. Pat. No. 4,539,122 describes use of erythorbic acid salts in combination with a molybdate salt, ferrous gluconate, and sodium gluconate. At more than 25 mpy (mili inches per year), the corrosion rate claimed for 18.5 pounds per gallon brines (2.20 g/cm3) at 300 degrees Fahrenheit (F), is too high for the most demanding applications. A somewhat similar inadequacy was noted with U.S. Pat. No. 4,980,074, which describes use of soluble aliphatic or aromatic aldehydes reacted with primary amines, and claims a less than satisfactory corrosion rate of 28 mils per year for a 19.5 pounds per gallon (2.34 g/cm3) brine at 250 degrees F. Only when supported by addition of sulfur compounds was the corrosion rate lowered to a level of 10-13 mpy with the aforementioned brine at 250 degrees F. U.S. Pat. No. 4,971,709 discloses use of some metallic powders as inhibitors for zinc containing brines. The main disadvantage of the method is the very low solubility of those powders, which cause precipitation of particles that might plug or damage a producing formation. Also U.S. Pat. No. 4,539,122, which proposes inhibiting heavy brines with arsenic containing compositions, is handicapped by the accumulative toxicity of arsenic compounds. WO 01/46552 claims use of heteropoly complex anions of transitional metal elements but omits to disclose the actual corrosion rates obtained in application simulations. EP 1038936 discloses the use of ammonia or amines in an amount effective to raise the pH as additives in inhibiting corrosion in brines; however, these additives can interfere with the stability of the brines. Formulas combining antimony with acetylenic alcohols have been proposed, for example in U.S. Pat. Nos. 4,498,997 and 4,522,658, for inhibiting oxidative effects of acidic aqueous environment.
In absorption refrigeration where lithium bromide solution is the preferred fluid, additions of zinc bromide improve the saturation concentration and provide a desirable low vapor pressure at absorber high temperatures. However, such solutions are extremely corrosive and cannot be used in the absence of very efficient corrosion inhibitors. Known additives include molybdates, chromates and nitrates. These additives lessen corrosion, albeit not to a satisfactory level, when significant amounts of zinc bromide are incorporated into the working fluids.
It is, therefore, an object of the present invention to provide a new corrosion inhibitor formulation for reducing corrosion induced by heavy brine fluids.
It is further, an object of the present invention to provide corrosion inhibition mixtures, which are compatible with different applications of working fluids, particularly at elevated temperatures.
It is further another object of the present invention to provide corrosion inhibiting mixtures which reduce corrosion rate to below 25 mpy when comprised in heavy brine fluids.
Another object of the present invention is to provide corrosion inhibiting mixtures which do not cause stress cracking to the apparatus in which they are used.
Still another object of the present invention is to provide use of combinations of corrosion inhibiting compounds in the manufacture of corrosion inhibiting mixtures.
Still another object of the present invention is to provide concentrated aqueous salt solutions that are strongly inhibited to prevent unacceptable levels of corrosion on the metallic apparatus in which they are used.
Still another object of the present invention is to provide concentrated aqueous salt solutions in which corrosion of steel is strongly inhibited.
Another object of the present invention is to provide strongly inhibited formulas of concentrated aqueous salt solutions that would prevent catastrophic sulfide stress cracking to the metallic apparatus in which they are used.
Another object of the present invention is to provide a method to use synergistic corrosion inhibitor formulations that would adapt the level of corrosion protection to the heavy brine fluids.
These and other objects of the present invention shall become clear as the description proceeds.